A large number of solutions enabling a thermally-activated short-circuiting switch to be connected in parallel with a battery cell have been proposed in the past.
In particular, some solutions take advantage of the fact that when the battery cell becomes faulty and becomes open circuit, a bypass diode is generally provided to allow current to continue flowing through the other battery cells that are connected in series with the faulty cell.
The flow of current through the diode causes the diode to heat up, and a switch of a first type makes use of this rise in temperature to establish a short circuit directly between the anode and the cathode of the diode.
Thus, for example, European patent EP-0 173 690 (Hughes Aircraft Company) proposes short-circuiting the electrodes of a bypass diode, either by causing a solder preform to run by a wick effect, thereby short-circuiting the contacts, or else by producing mechanical deformation of an electrode pressed against the diode and serving to establish a short circuit with another electrode on the periphery of the diode.
U.S. Pat. No. 3,213,345 (Mallory) proposes a bypass diode having an electrode urged resiliently towards the periphery of its package in a short-circuit position and soldered to the diode by solder which is caused to melt by a high current passing through the diode, thereby establishing the desired short circuit.
Finally, German patent DE-1 613 968 (Brown Boveri) proposes a device comprising two anti-parallel diodes which device is short-circuited by an alloy melting in the event of bypass current flowing, the short circuit taking place in a cavity situated in the bottom portion of the diode and producing a short circuit at the periphery thereof.
Each of the solutions described above suffers from the drawback of depending on the particular shape of the electrodes of the diode. Thus, European patent EP-0 173 690 establishes a short circuit on a ring constituting the outside of the diode, which means that it is difficult to obtain contact that is reliable, having low resistance, and enabling a high nominal current to pass. U.S. Pat. No. 3,213,345 provides contact that is very small only, since the resilient electrode soldered to the diode cannot be very large in size. Finally, the solution proposed in German patent application DE-1 613 968 also depends closely on shape, in particular of the diode, in order to be able to achieve sealing around the periphery thereof, and it also implies that the diode remains in a vertical position since flow takes place by gravity. Such a solution is unsuitable for use on board a satellite, in particular.
In European patent application EP-0 226 360 (Powerplex Technologies) a switch is described that is similar to the first above-specified type and that uses a zener diode in parallel with a battery cell. In the event of the battery cell failing, the battery current flows through the zener diode by melting it, providing the package of the diode is not damaged. This short-circuiting makes use of a mechanism that is not well understood, thereby making it difficult in practice to control the value of the contact resistance, and in particular the reproducibility thereof.
From the above, it results that however attractive it may appear, implementing short-circuit switches of the first type by producing a direct short circuit between the electrodes of a bypass diode suffers from drawbacks and/or limitations in practical implementation that are quite severe.
A second type of switch makes a short circuit directly across the battery cell.
PCT application WO 88/00400 (Hughes Aircraft Company) thus proposes using an electrode that is soluble in the electrolyte of the cell. That solution turns out to be difficult to implement, since the desired short circuit is obtained by nickel being deposited on the electrode. In addition, the resistance of the short-circuit contact and the possibility of allowing a high current to pass are not guaranteed.
A third type of switch achieves a short circuit without directly short-circuiting the contacts of bypass diodes. Such switches, which may optionally be connected to the electrodes of a bypass diode, are also described in a certain number of prior publications.
U.S. Pat. No. 5,025,119 (Hughes Aircraft) describes a short-circuit switch implementing a self-solderable resilient blade contact controlled by an electromagnetic coil. This implies that a sensor detects faulty operation of the battery cell and activates the electromagnetic coil. In other words, the operation of that device depends on the reliability of an external circuit which gives rise to qualification problems for a system on board a satellite which needs to be effective for very long missions, e.g. exceeding five years, and possibly as long as fifteen years.
European patent application EP-0 372 823 (Hughes Aircraft) describes a short-circuiting device connected in parallel with a bypass diode of a battery cell and controlled by a thermal switch which itself uses a relay to actuate a main contact capable of passing all of the current flowing through the battery cells connected in series with the faulty cell. As in the preceding case, implementation depends on the reliability of several electronic components.
U.S. Pat. No. 4,061,955 (NASA) describes a short-circuiting circuit comprising a fault-detecting semiconductor device coupled with a relay. As before, that technique suffers from drawbacks of reliability associated with having an electronic circuit for detecting a fault.
Finally, U.S. Pat. No. 4,252,869 (Dow Chemical) describes a device for short-circuiting two electrodes, a central electrode and an electrode disposed concentrically thereabout, in the event of heating caused by current passing because of a faulty battery cell breaking an ampoule containing a conductive liquid which forms a short circuit between the two above-mentioned electrodes. That device can operate only under gravity, and it is not usable in weightlessness on board a satellite.